Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers.
Particularly diazo-sensitized systems are widely used. These systems have been extensively reviewed by Kosar J. in "Light-Sensitive Systems", Wiley, New York, 1965, Chapter 7. A generally used negative-working diazo-sensitized system is based on the capability of diazo compounds to harden a polymer when exposed to ultraviolet and blue radiation. Diazo compounds which have been used for the preparation of lithographic printing plates based on their hardening properties are e.g. diazonium salts whose photolysis products can harden polymers (natural colloids or synthetic resins) directly and diazonium polymers. Although polymers containing diazonium groups have a large structure they remain water soluble owing to the presence of the ionic diazonium groups. When these groups are destroyed by exposure to light an insoluble resin is formed. Particularly useful diazonium polymers are the condensation products of a carbonyl compound, e.g. an aldehyde, such as formaldehyde, with a diazonium salt of e.g. a p-aminodiphenylamine. These condensation products are usually designated diazo resins. In these systems a polymeric binder is optionally added to the diazo resin coating.
U.S. Pat. No. 3,549,373 discloses a multi-layer reversible copy sheet on which a positive image is obtainable from a negative original, wherein said sheet comprises a non-metallic base, an intermediate layer on said base, and a light-sensitive diazo compound-containing layer on said intermediate layer, wherein said intermediate layer is formed of a resin that comprises a water-soluble alkali metal salt of a copolymer of a vinyl compound, and an unsaturated organic carboxylic acid.
U.S. Pat. No. 5 466 557 discloses a radiation-sensitive composition comprising (1) a resole resin, (2)a novolac resin, (3) a latent Bronsted acid, (4) an infrared absorber, and (5) terephthalaldehyde.
DE-A- 4 241 717 discloses a positive working light-sensitive composition, characterized in that it comprises a diazonium salt and an alkali-soluble polymer.
However there exist no positive working printing plate obtained from a photosensitive element containing diazonium salts as light sensitive substance. Most actual positive working printing plates are obtained from photosensitive elements using chinone diazide derivatives as light sensitive substance. Chinone diazide derivatives are difficult to synthesize and therefore expensive where diazonium salts are easier to synthesize and herefore cheaper. So, there is a long-felt need for photosensitive elements containing diazonium salts which yield positive printing plates.